SU‐E‐J‐86: A Deformable Image Registration Method for Dose Accumulation Between IMRT and HDR CT Images

Purpose: To develop and validate a method to improve existing deformable image registration (DIR) algorithms for the registration between the high‐dose‐rate (HDR) brachytherapy CT image and the intensity‐modulated radiation therapy (IMRT) CT image of gynecologic cancer patients for dose accumulation. Methods: Deformable registration of brachytherapy CT images and external beam CT images is challenging due to the presence of the applicator in the brachytherapy image. We have developed a method to adapt DIR algorithms to this situation. A segmentation step is used to remove the applicator, which is imaged in the HDR CT image but not in the IMRT image, leaving an empty virtual cavity. An artificial deformation vector field is then generated in the tissue around the applicator to deflate this cavity toward the center at each slice and restore the vagina to a state without applicator. Different schemes are employed for two different registration tasks, i.e., deforming HDR CT image to IMRT CT image, or vice versa. In both cases, demons algorithm is used as the main DIR method to perform registration. The generated final deformation vector fields are used to deform the dose grid from one CT anatomy to another for dose accumulation. Results: Visual inspection of the DIR results indicates that our proposed method can suppress the interference of the applicator to the DIR algorithm, and successfully and accurately register the HDR and IMRT CT images as well as deforming and adding doses from both treatment modalities. Conclusion: We have developed a novel and robust DIR scheme that can perform registration between IMRT CT image and HDR CT image for dose accumulation, and can yield accurate registration results. This work was supported in part by the National Natural Science Foundation of China (no 30970866), Major Program of Natural Science Foundation of Guangdong Province of China (no 10251051501000007).